In the transportation industry, there is considerable impetus for the reduction of weight of vehicle components. In many cases, for example, the reductions in weight are necessary to achieve designated fuel economy standards that are becoming even more stringent. Alternative designs and/or constructions of many vehicle components are often considered particularly in the automotive sector as well as in other transportation industries if the resulting parts can achieve significant weight savings.
Recently, some automakers have been exploring the use of microcellular foaming technologies for molding lighter weight plastic substrates for overall vehicle weight reduction and improved fuel economy. Plastic substrates that are formed using microcellular foaming technologies (e.g., microcellular polymeric substrates) typically include a microcellular structured core that is sandwiched between two outer skins all of which are formed during a molding process. In one example, during an early stage of the molding process, carbon dioxide, nitrogen and/or the like is introduced or released into a polymer melt to produce a low viscosity multi-component polymer melt that is injected into a molding tool. When the low viscosity multi-component polymer melt contacts the cooler metal surfaces of the molding tool that define a molding cavity, a boundary layer(s) of solid polymeric material is rapidly formed, e.g., rapidly freezes or solidifies, along the cooler metal surfaces to form the outer skins. The remaining space in the molding cavity between the outer skins is progressively packed with the low viscosity multi-component polymer melt, which develops microscopic-sized bubbles or voids (e.g., of carbon dioxide and/or nitrogen) in the polymeric material while cooling and more gradually solidifying to form the microcellular structured core. The microscopic-sized bubbles or voids help reduce the total density of the microcellular polymeric substrate for overall weight reduction.
Some vehicle parts are formed by welding (e.g., vibration welding, ultrasonic welding, friction welding, or the like) two or more plastic substrates together to form a welded plastic panel. Examples of such vehicle parts include interior or exterior trim components and/or structural components, such as instrument panels with integrated airbag systems, door trim panels and modules, consoles, defroster ducts, and knee bolster and/or glove box door assemblies. Unfortunately, welding two or more plastic substrates together when one or more of the plastic substrates is a microcellular polymeric substrate can produce a plastic panel that has less robust and/or lower weld strength weld joints than traditional plastic panels that are formed from welding two or more solid plastic substrates together. In particular, it is believed that the microcellular structured core of the microcellular polymeric substrate is relatively compliant and compresses during the welding process in response to welding pressure that is applied to join the two plastic substrates together. This effectively decreases the resulting welding pressure that otherwise should be relatively high to form robust and/or high weld strength weld joints. This can be problematic particularly in relatively high energy, high stress, and/or high impact plastic panel applications, e.g., instrument panels with integrated airbag systems, knee bolsters, and the like, where robust and/or higher weld strength weld joints may be needed to avoid partial or full delamination of the plastic substrates.
One conventional approach to improving the weld strength of weld joints of plastic panels is disclosed in U.S. Pat. No. 8,025,946, issued to Fujita et al. In Fujita, a vibration-welded structure including two plastic parts that are joined together by vibration welding is provided. Each of the two plastic parts has a welding rib with a corresponding welding surface. The welding ribs are each provided with a guide portion capable of guiding the movement of the other welding rib in a vibration direction. This arrangement helps improve the welding strength of welding joints formed between the two parts in cases where the angle between the welded surfaces and the vibration direction becomes relatively large by focusing the welding pressure on the welding surfaces. Unfortunately, the guide portions do not effectively increase the welding pressure along the welding surfaces in cases where one or both of the plastic parts have a relatively compliant core that compresses in response to the applied welding pressure, e.g., microcellular polymeric substrate.
Another conventional approach to improving the weld strength of welding joints of plastic panels is to increase the welding surface area for forming the welding joints. Unfortunately, this is not always practical because package space is often limited in many applications and increasing the welding surface area may not be a viable option.
Accordingly, it is desirable to provide plastic panels for motor vehicles that include a microcellular polymeric substrate welded to a polymeric substrate for overall weight reduction and that have robust and/or relatively high weld strength weld joints, and methods for making such plastic panels. Moreover, it is desirable to provide plastic panels for motor vehicles that include a microcellular polymeric substrate welded to a polymeric substrate that can be accommodated in relatively limited package space, and methods for making such plastic panels. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.